After several decades of evolution, e.g., from 2G, 3G and 4G, and now approaching 5G, mobile networks are able to provide billions of mobile users with data transmission service via almost ubiquitous radio access. Different generations of mobile networks have distinguished features, technologies, and even network architectures and protocol stacks. In order to protect the investment of both operators and end users in prior generation technologies, the introduction of each new generation network has supplemented but not replaced previous generation networks. Thus, old and new generation networks co-exist with one another and will continue to co-exist for many years to come. For example, many mobile networks today consist of Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS) and Long term Evolution (LTE) systems. Similarly, handsets, or other user equipment (UE) often support multiple modes, each mode utilizing a different RAT.
Though a UE with the capability of supporting multiple modes can communicate using different RATs, only one RAT is active at any given time. In other words, data is transmitted using only one RAT at a time. And if the active RAT cannot meet the demands of the service, then an inter-RAT handover will typically occur.
FIG. 1 illustrates an exemplary state diagram 100, which shows how Radio Resource Control (RRC) status changes with the handover between 3GPP 2G/3G/4G RATs. From a UE's perspective, each RAT works on its own basis independently. For example, in some networks, a voice call via a GSM connection may disconnect a UE from a 4G connection until the voice call ends. The coordination among multiple RATs may only take place at the moment when an inter-RAT handover is intended, even though in many cases, a cell site might support different RATs at the same time due to the limitations in resources, e.g., site acquisition and maintenance costs, etc. Actually, an end user does not need to consider whether it connects to a 2G, 3G or 4G etc. network. The concern of the end user is related to whether the wireless network can provide data services on demand, regardless of the generation of the networks on which the data services are provided.
As illustrated in FIG. 1, RRC is a Radio Resource Control protocol used by RATs such as UMTS and LTE to provide the Air interface for wireless communications. RRC handles the control plane signaling of Layer 3 between the UE and the Radio Access Network (e.g., UTRAN or E-UTRAN) as well as for the radio interface between a Relay Node and the E-UTRAN. This RRC protocol is specified by 3GPP Technical Specification TS 25.331 for UMTS and Technical Specification TS 36.331 for LTE, both of which are incorporated herein in their entireties. RRC messages are typically transported via the Packet Data Convergence Protocol (PDCP).
The primary functions of the RRC protocol include connection establishment and release functions, broadcast of system information, radio bearer establishment, reconfiguration and release, RRC connection mobility procedures, paging notification and release and outer loop power control. By means of the signaling functions, the RRC configures the user and control planes according to the network status and allows for Radio Resource Management strategies to be implemented. The operation of the RRC protocol is typically guided by a state machine which defines certain specific states that a UE may be present in. The different states in this state machine have different amounts of radio resources associated with them and these are the resources that the UE may use when it is present in a given specific state. Since different amounts of resources are available at different states the quality of the service that the user experiences and the energy consumption of the UE are influenced by this state machine.
As illustrated in FIG. 1, exemplary E-UTRA states include a RRC connected state 102 and a RRC idle state 104. The states of the RRC connected state 102, in order of decreasing power consumption, are: a CELL_DCH (Dedicated Channel) state 106, a CELL_FACH (Forward access channel) state 108, and CELL_PCH (Cell Paging channel)/URA_PCH (URA Paging channel) state 110. For example, the power consumption in the CELL_FACH state 108 can be roughly 50 percent of that in the CELL_DCH state 106, and the PCH states 110 use about 1-2 percent of the power consumption of the CELL_DCH state 106. The RRC idle state 104 (i.e., when there is no active connection with a network resource) has the lowest energy consumption and, in the example shown in FIG. 1, includes an UTRA Idle state 112 and a GSM Idle/GRPS Packet Idle state 114. The transitions to lower energy consuming states occur when inactivity timers trigger. For example, a first timer (T1) controls transition from the DCH to FACH state, a second timer (T2) controls transition from the FACH to PCH state, and a third timer (T3) controls transition from the PCH to idle state. Different operators can have different configurations for the inactivity timers, which leads to differences in energy consumption.
In the RRC Idle state 104, a UE can only be located by its tracking area (TA) within the network coverage area, which means that the network is unaware of a specific base station the UE is currently assigned to. After a RRC connection procedure is completed, the UE transitions to the RRC Connected state 102, after which the UE may use dedicated network resources to perform traffic data transfer functions. After completion of data transfer, the UE will transition back to the the RRC Idle state 104 in accordance with a predetermined RRC Connection Release procedure in order reduce energy consumption by the UE. In the example shown in FIG. 1, in the RRC Connected state 102, the UE can transfer data in either a Global Satellite Mobile (GSM) Connected state 116 (utilizing a GSM RAT) or a General Packet Radio Service (GPRS) Packet transfer mode state 118 (utilizing a GPRS packet transfer mode RAT). In a conventional UE or base station that supports duel connectivity, however, only one RAT can be used at any given time.
Dual connectivity introduced in networks such as 3GPP Release 12 (R12) supports operations where a given UE can consume radio resources provided by at least two different network points (e.g., Primary and Secondary eNodeBs), typically connected with a non-ideal backhaul, while in a RRC_CONNECTED state. Thus, the UE can be provided with higher data throughput via a radio bearer split which means the radio bearer is split among multiple E-UTRAN node B's (a.k.a., “evolved Node B” or “eNodeB”). Thus, in conventional networks, dual connectivity is provided by utilizing multiple eNodeBs that operate using the same radio access technology (RAT), e.g., LTE. Additionally, when dual connectivity is provided, the data stream is split at the radio bearer. These techniques result in inefficiencies in utilizing the different RAT's that are supported by networks and UE's today.
FIG. 2 illustrates an Open Systems Interconnection (OSI) model of a conventional UE protocol stack 200, which includes a control plane 202 and a user plane 204. The control plane 202 provides OSI Layer 3 signaling between the UE and the Radio Access Network (UTRAN or E-UTRAN) and includes a Non-Access-Stratum (NAS) layer 206, which controls session management, mobility management and security management. In various embodiments, NAS messages may be transported by the Radio Resource Control (RRC) layer 208 either by being concatenated with other RRC messages or encapsulated as dedicated RRC messages. The RRC layer 208 may be terminated by the eNodeB for 4G network, Radio Network Controller (RNC) for 3G network etc., and in various embodiments, the RRC layer 208 controls system information broadcast, paging, RRC connection between the UE and the network, and point-to-point radio bearers. In various embodiments, the RRC layer 208 is also involved in various mobility functions including but not limited to: UE measurement reporting and control of the reporting for inter-cell, Inter-RAT mobility, UE cell selection/reselection, etc.
The user plane 204 includes an Application (APP) layer 210 and an Internet Protocol (IP) layer 212. The APP layer 210 is the OSI layer closest to the end user operating UE, which means both the APP layer 210 and the user interact directly with a software application running on the UE. Thus, the APP layer 210 interacts with software applications that implement one or more communicating function such as identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the APP layer 210 determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the APP layer 210 decides whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the APP layer 210. Thus, the APP layer 210 supports application and end-user processes. The IP layer 212 provides the principal communications protocol for relaying data across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. The IP layer 212 has the task of delivering packets from the source host to the destination host solely based on the IP addresses in the packet headers. For this purpose, IP layer 212 defines packet structures that encapsulate the data to be delivered. It also defines addressing methods that are used to label the data with source and destination information.
A PDCP layer 214 provides control plane data to the RRC layer 208 and user plane data to the IP layer 212 of the UE. The PDCP layer 214 can also provide data to base stations (e.g., eNodeBs). The PDCP layer 214 further includes a header compression sublayer or module 216 for providing header compression services to upper layers, a ciphering module 218 for providing ciphering services to the upper layers, and an integrity module 220 for providing data integrity services to the upper layers. The header compression module 216 can utilize known IP header compression protocols (e.g., RFC 2507 or RFC 3095). If PDCP is configured for No Compression it will send the IP Packets without compression; otherwise it will compress the packets according to its configuration by upper layer and attach a PDCP header and send the packet. Different header formats are defined, dependent on the type of data to be transported. The ciphering module 218 ciphers IP data to be sent to the upper layer utilizing known ciphering techniques. The integrity module 220 performs known data integrity and ciphering functions on control messages sent to the RRC layer 208.
Referring still to FIG. 2, a Radio Link Control (RLC) layer 222 transports the PDCP's protocol data units (PDUs). The RLC layer 222 includes a segmentation module 224 that segments each data packet adaptive to the RAT being supported, and supports Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM) in various embodiments. For various AM mode embodiments, Automatic Repeat ReQuest (ARQ) is applied for guarantee of data segmentation transmission. The RLC layer 222 further includes an ARQ module 226 that provides error-control for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a data frame or packet) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission. If the sender does not receive an acknowledgment before the timeout, it usually re-transmits the frame/packet until the sender receives an acknowledgment or exceeds a predefined number of re-transmissions. Depending on the mode, the RLC layer 222 can provide: ARQ error correction, segmentation/concatenation of PDUs, reordering for in-sequence delivery, duplicate detection, etc.
The UE protocol stack 200 further includes a first Medium Access Control (MAC) layer 228n configured for RAT #n, which schedules uplink/downlink data transmission, and performs link adaptation, random access control by means of a first multiplexing module 230n, and makes error correction by means of a first Hybrid Automatic Repeat ReQuest (HARQ) module 232n, in accordance with the RAT #n protocol. The UE protocol stack 200 fur includes a second Medium Access Control (MAC) layer 228k configured for RAT #k, which schedules uplink/downlink data transmission, and performs link adaptation, random access control by means of a second multiplexing module 230k, and makes error correction by means of second Hybrid Automatic Repeat ReQuest (HARQ) module 232k, in accordance with the RAT #k. The MAC layer 228 provides addressing and logical channels to the RLC layer 224 that make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium.
The UE protocol stack 200 further includes multiple physical (PHY) layers 234k and 234n, each corresponding to a supported RAT #k and #n. The PHY layer 234 is responsible for the actual transmission over the radio interface and includes a coding and modulation module 236, including channel coding, modulation and the physical signal generation for transmission via physical channels. Note that in the protocol stack 200 of FIG. 2, the mapping between logical channels and transport channels is a 1-to-1 mapping, which is one of the reasons that only one RAT can be active at any one time.